Fully-differential amplifiers suitable for use as operational amplifiers (“op-amps”) are one of the main building blocks in analog circuit design. Op-amps are used in many applications, such as analog-to-digital converters (ADCs), digital-to-analog converters (DACs), voltage reference generators, and sample-and-hold circuits. One example op-amp configuration from the prior art is shown in FIG. 1. Input signals 102 and 104 to op-amp 101 and output signals 106 and 108 from op-amp 101 may each include both a differential and a common mode (CM) signal. Op-amp 101 has a closed-loop structure, such that output signals 106 and 108 are related to the input signals 102 and 104. The common-mode (CM) voltage of the output signals 106 and 108 are set by a constant reference voltage VCM—ref applied as a common mode reference input signal 110. A common-mode variation at the input signals 102 and 104 is replicated as the common-mode input of the op-amp 101. This variation can degrade the performance in some applications, such as by reducing linearity of the op-amp 101. Some conventional solutions to this variation are described with reference to FIG. 2 and FIG. 3.
FIG. 2 is a circuit in accordance with prior art U.S. Pat. No. 6,603,356. The fully differential op-amp 101 of FIG. 1 is substituted with two differential-input single-ended output op-amps 201 and 202. The non-inverting input of the op-amps 201 and 202 are connected to a constant common-mode (CM) voltage Vcm. The Vcm signal maintains the input common-mode at a constant level to reduce the variation mentioned above. However, this conventional solution has at least two drawbacks. A first drawback is the use of two op-amps 201 and 202 that results in at least 3 dB additional noise compared to use of the single op-amp 101 of FIG. 1. A second drawback of this architecture is that by replacing one op-amp 101 with two op-amps 201 and 202, the total power and area consumption of the chip is larger.
FIG. 3 is a circuit of another conventional solution to the variation described above. A circuit 300 includes an input common-mode node that is sensed by DAC 302. The detected common-mode at the DAC 302 input is compensated by outputting signals to op-amp 301 from current sources embedded inside the DAC 302. One drawback of the circuit 300 is additional noise introduced by digital-to-analog converter (DAC) 302.
Shortcomings mentioned here are only representative and are included simply to highlight that a need exists for improved electrical components, particularly for operational amplifiers (or “op-amps”) employed in consumer-level devices, such as mobile phones. Embodiments described herein address certain shortcomings but not necessarily each and every one described here or known in the art. Furthermore, embodiments described herein may present other benefits than, and be used in other applications than, those of the shortcomings described above.